add a notifications helper class

[xaoseric]

This adds a notifications helper class, should help with implementing issue #951

[mstarke]

Thank you for the PR but this is currently not mergeable for 0.7.x since the UserNotification framework requires 10.14 and the deploy target for MacPass ist still 10.10. This will be raised with 0.8 to 10.12 or even higher but the 0.7.x branch should stay compatible with 10.10. which requires to stick with the deprecated NSUserNotification. Another note - I'm hesitant to introduce .swift code since it requires to bundle the swift runtime AFAIK and I also do not have no plans to fully convert MacPass to swift any time soon. This might sound foolish but for now I'm more productive with Obj-C and keeping the code base manageable for me is of high value.